In general, there are two types of printed wiring board used for application to the livelihood instruments such as televisions, radios, tape recorders, stereo instruments, precise devices such as cameras, watches and metering instruments, and other electro and electronic devices. One of the printed wiring boards is a one-side printed wiring board having an electrically conductive circuit layer provided on one of the sides of an insulating laminate. Another is a two-sided printed wiring board having two electrically conductive circuit layers provided one on each side of an insulating laminate and through-holes provided thereacross.
The one-side printed wiring board has been normally produced by printing, in a predetermined pattern, an etching resist on a copper foil on a one-side copper foiled insulating laminate, then etching that portion of the copper foil uncovered by the etching resist and thereafter removing the etching resist from the residual copper foil whereby a predetermined electrically conductive circuit layer is formed.
The two-sided printed wiring board has been normally produced in a manner as follows; through-holes are formed in a two-sided copper foiled insulating laminate; and then electroless copper and electrical copper plating layers are formed on the walls of the through-holes so as to permit the copper foils to be electrically connected by the plating layers. Thereafter, plating resists are painted on the copper foils in a pattern reverse to a predetermined pattern, and those portions of the copper foils uncovered by the plating resists are plated with electrical solder plating layer. Then, the plating resists are removed from the residual copper foils, after which the copper plating layers on the wall of the through-holes and the residual copper foils are etched with the electrical solder plating layer as etching resists.
As noted from the foregoing, the one-side printed wiring board can be inexpensively massproduced because its manufacturing process is simple, and easily assembled because of its having an electrically conductive circuit layer provided on one side thereof. However, it cannot advantageously follow the requirement of smaller scale, higher performance and higher density of instruments which is accompanyed by a recent rapid development of the electronic or electric instruments. The one-side printed wiring board cannot be applied for the instruments having a high density of components included therein.
The two-sided printed wiring board can be applied for such high density instruments, but it fails to be inexpensively mass-produced because its manufacturing process is complicated, and in addition thereto, in order to produce such wiring board of high reliance, insulating layers of epoxy resin impregnated woven glass fiber laminate are to be used in view of lower heat expansion of the insulating layers, while electrically conductive copper plating layers are to have the thickness of more than 25 micron.
As the requirement of higher density of components increases, even the two-sided through-hole type printed wiring board cannot be applied for such requirement. Furthermore, there have to be solved various problems such as complicated assembly which is caused by the electrically conductive circuit layers provided on both sides of the board and electrically insulating treatment after its assembly.